Theresa May has expressed her sympathies to the victims of the "evil and cowardly" Westminster terror attack carried out by Khalid Masood.
The Prime Minister added her handwritten card to piles of tributes outside Parliament.
It said: "With deepest condolences for those who lost their lives as a result of this evil and cowardly attack. Our thoughts and prayers are with them and their families."
Kurt Cochran and his wife Melissa had been visiting London from the US to celebrate their 25th wedding anniversary and were due to fly home.
A 30-year-old man was arrested in Birmingham on Sunday on suspicion of preparing terrorist acts.
Dozens of women gathered on Westminster Bridge to form a human chain in a show of solidarity with the victims of Wednesday's terror attack.